In recent years, wide-area Ethernet (registered trademark) and IP-VPN (internet protocol-virtual private network) have been becoming increasingly advanced and widespread, and environments where WAN (wide-area network) is realized by an IP network in place of a conventional dedicated line or ATM (asynchronous transfer mode) have been increasing. Accordingly, voice service, which has conventionally been realized by PSTN (public switched telephone network) using PBX (private branch exchange), has come to be realized by VoIP using VoIP (voice over IP) gateway or a VoIP adapter. In particular, SIP (session initiation protocol) based on text descriptions with a high affinity for HTTP (Hypertext Transfer Protocol) has becoming more common as a signaling protocol for realizing voice service using VoIP as compared to H.323 whose data structure is complicated.
In SIP environments, SIP servers are commonly placed with an SIP terminal, and SIP servers offer functions such as registration and name resolution of an SIP server address, and call routing. SIP servers may be placed in a public network of an ISP (Internet service provider), or placed in a private network of a company.
SIP servers are broadly classified into three types according to functions. The first SIP server is an SIP registration server that carries out registration and name resolution of an SIP server address. The second SIP server is an SIP proxy server that acts as an alternative to an SIP terminal to carry out SIP communication with an opposite SIP terminal and a VoIP gateway. The third SIP server is an SIP redirect server that sends back an opposite destination that carries out call routing suitable for an SIP terminal. There is known a network system in which these SIP servers are placed separately for individual functions (see, for example, PTL (Patent Literature) 1).
When different SIP servers are placed for different functions, a normal response cannot be obtained from an SIP server unless an appropriate SIP message (an SIP message suitable for the type of the SIP server) is transmitted from an SIP terminal to an appropriate server. For example, an error is caused by transmission of a message requesting the start of SIP communication to an SIP registration server, and also, an error is caused by transmission of a message registering an SIP server address to an SIP proxy server. For this reason, SIP communication may not be carried out unless appropriate SIP server settings are configured at an SIP terminal.
If settings including the types of SIP servers are manually configured, the above problem can be avoided because appropriate SIP servers can be manually set one by one.